For a number of reasons, it is highly desirable during the setting of blind fasteners, such as blind rivets, to monitor the load being applied to the fastener during the setting operation and comparing the determined load values against predetermined reference values to monitor and analyse the quality of the setting operation in order to provide a confidence factor that the fastener has been correctly set. This is particularly advantageous since such blind fasteners are often used in situations where it is difficult to visually confirm acceptability of the set fastener (ie. the “blind side” is often located in an internal surface of a sealed box or container which cannot be viewed by the operator).
Conventional setting tools for use with blind fasteners work on the principle of providing a front end surface of a setting tool to restrain the flange portion of the blind fastener and for a mandrel stem of the blind fastener to pass therethrough to be engaged by a set of pulling jaws whereby such pulling jaws are drawn inwardly for the setting tool to exert a displacement force or setting force on the mandrel stem to effectively draw a mandrel head into the fastener body to deform the free end of the fastener against an appropriate work surface. Often a hydraulic or pneumatic force is utilised to effect displacement of the pulling jaws and the measurement of the force applied to the mandrel stem is determined by analysis of the pressure applied by the hydraulic or pneumatic fluid, usually via a pressure transducer, used to drive a uniform piston attached to such jaws. The applicant's earlier European Patent No. EP0 738 8551 discloses such a conventional load measuring system for a blind rivet setting tool.
It has also been known to utilise conventional strain gauges interconnected between the tool body and the setting jaws to again measure the load exerted on the fastener dependent on the displacement of the jaws themselves.
Whilst highly effective in determining the load transferred from the setting jaws of such conventional rivet setting tools to the mandrel of this fastener during the setting operation, these types of existing load measuring systems are highly complex and require careful positioning internally of the rivet setting tool making manufacture of such tools difficult and the repair and replacement of worn or damaged measurement devices extremely difficult.
It is therefore an object of the present invention to provide a blind fastener setting tool having an improved load measuring device which alleviates the aforementioned problems in a simplistic and cost effective manner.
According to the present invention there is now provided a blind fastener setting tool having a front face against which a blind fastener is held during a setting operation, and having a piezo-electric thin film load measuring device mounted on the front face so as to be disposed and compressed between the front face and the fastener during the setting operation. In this manner, the compressive force exerted by the fastener on the load measuring device will generate a low voltage signal indicative of the load being exerted thereon during the setting operation.
Preferably, the end face will be mounted on the tool by a bridge member so as to form a cantilever which is subject to bending when a load is exerted on the fastener by the setting tool. Here the load measuring device will usually comprise a bending piezo-electric generator which is securely mounted on the front face, wherein the bending deformation of the generator, resultant from the bending deformation of the cantilever, will generate a low voltage electrical signal.
Preferably, the front face of the setting tool will have a central aperture therethrough providing communication with the internal mechanism of the tool, this aperture being coaxial with a longitudinal axis of the setting tool and being for receipt of a mandrel of the fastener, wherein the load measuring device further comprises an aperture so as to be mounted again coaxial with the tool axis.
It is also preferable that a protective cover is mounted on an external surface or face of the measuring device to protect the thin film piezo-electric material from mechanical damage from engagement with the fastener.
Further and according to the present invention there is also provided a system for measuring the load exerted on a blind fastener by a fastener setting tool during a setting operation which comprises a setting tool as discussed above and further has a control circuit for analysing a voltage output of the piezo-electric thin film load measuring device as indicative of the load exerted from the fastener.
Still further according to the present invention, there is also provided a method of measuring the load exerted on a blind fastener by a fastener setting tool during a setting operation which comprises the steps of firstly positioning a piezo-electric thin film load measuring device between a front end face of the tool and a fastener mounted on the tool, subsequently compressing the fastener towards the end face during the setting operation so as to compress and deform the measuring device, following which a voltage signal created as a result of deformation of the piezo-electric thin film is measured, and this measured signal is then analysed as indicative of the load exerted on the fastener.
Preferably, where the load measuring device is mounted on a cantilevered front end face of a setting tool such that this end face is caused to bend as a compressive force is applied thereto, the deformation of the piezo-electric thin film comprises a bending deformation to generate the electrical signal.
There is further provided a method of determining a free set operation of a fastener setting tool by measuring the load exerted on a blind fastener comprising the step of determining the measured time difference between the mandrel entry load and the mandrel setting load of such fastener and comparing it against a predetermined time difference value indicative of an optimum setting time difference and generating an output signal in the event that the measured time difference is greater than the predetermined time difference indicative of a free set operation.